Method for forming a layer of refractory fibers on a surface and material produced thereby

ABSTRACT

A refractory binder including a tackifying agent. The tackifying agent is a montmorillonite clay. The binder is a chromium alumina phosphate mixture with a fluxing agent. The binder may also be a colloidal suspension of alumina, silica or zirconia with montmorillonite clay. The binder is used with refractory fiber to produce a monolithic layer.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application, Ser. No.06/660,190 filed Oct. 12, 1984 U.S. Pat. No. 4,737,192 which is acontinuation-in-part of patent application Ser. No. 06/542,272 filedOct. 17, 1983 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a high temperature inorganic binder. Moreparticularly, this invention relates to an inorganic chemical binderwhich may be used in conjunction with refractory fibers at very hightemperatures. In still greater particularity, the invention is aninorganic binder including a tackifying agent. By way of furthercharacterization, but not by way of limitation thereto, the invention isa chromium aluminum phosphate binder including bentonite clay.

2. Description of the Prior Art

Prior art binders for bonding refractory fibers have included a varietyof compounds including colloidal silica and phosphoric acid (actually P₂O₅ which may be derived from H₃ PO₄ under appropriate temperatureconditions). While these binder systems are useful in certainapplications, they are not without relative disadvantages. For example,prior art phosphoric acid binders do not adhere well to refractoryfibers. Binders including colloidal silica are subject to irreversibleprecipitation of the silica if the binder has been subjected to freezingtemperatures.

A very serious disadvantage with prior art binder systems is bindermigration. That is, as the binders are cured, the binder migrates to thesurface of the material producing a material with a very soft interior.This causes serious integrity problems with the resulting material. Thematerial is thus not suitable for use in applications requiring amonolithic structure. In addition, prior binder systems employingcolloidal suspensions such as colloidal silica or alumina must be curedvery carefully. That is, because of binder migration, the colloidal solmust be allowed to gel prior to curing. Failure to do so results inmigration of the binder particles from the interior to the surface ofthe insulation layer. In addition, prior art binders must be sprayedonto a cool surface thus necessitating relatively long cooling periodsbefore a furnace can be insulated. Another disadvantage of prior artbinders has been that they are not nearly as temperature resistant asthe refractory fibers themselves. Thus, high temperature applications ofthe fiber-binder mix are limited by the properties of the binder ratherthan the fiber. For example, an insulation product made from refractoryfibers which are themselves temperature resistant up to 2600° F. mayonly be applied in temperature environments up to 2000° F. if the binderused in that product is only effective up to 2000° F. Any reaction ofthe binder and fiber which tends to flux the fiber composition causesincreased fiber shrinkage. Thus, the use temperature of thefiber--binder system is limited to a temperature less than the usetemperature of the fibers alone.

Other practical problems encountered with the use of prior bindersystems is that they are expensive, there may be safety andenvironmental problems in their applications, they may lack goodsuspension properties, and finally, as with prior phosphoric acidbinders, the binders do not possess the tackiness or adhesivenessproperties which are desirable in many applications. In applicationswhere it is desirable to spray the refractory fiber onto a surface suchas a furnace wall, it is essential that the binder be sufficiently tackyto cause the fibers to adhere to one another and to the surface.

SUMMARY OF THE INVENTION

The present invention combines an inorganic binder matrix withtemperature resistant clays. The tacky nature of the binder renders itsuitable for a wide variety of applications. The resulting binder of thepresent invention is temperature resistant to at least 3000° F. whichallows its use with high temperature refractory fibers. As used herein,refractory fibers are defined as inorganic amorphous or crystallinefibers which are not adversely affected by temperatures in excess of1500° F. An example of such fibers in alumina-silica fibers. However, asis known in the art, fibers including zirconia, chromium, calcium,magnesium and the like may be utilized. The binder disclosed hereincould also be employed with metal oxides to provide an insulating layerof such material. In addition, the binder system of the presentinvention greatly reduces or eliminates binder migration which occurs inprior art binders.

The binder includes a tackifying agent in combination with a phosphatebinder matrix such as aluminum phosphate. The preferred tackifyingagents are temperature resistant clays such as montmorillonite clays.The most preferred clay is western bentonite clay. A fluxing agent mayalso be added to the binder in small quantities to increase the strengthof the binder to fiber bonds at elevated temperatures. Chromium oxidemay also be added to the binder to improve the stability of the binderto fiber bonds at elevated temperatures. In addition, chromium oxideadds color to the binder which may be advantageous in the coatingprocess.

In addition to phosphate compounds, colloidal suspensions such ascolloidal silica, colloidal alumina, and colloidal zirconia may becombined with the tackifying agents disclosed herein to produce asuitable binder. When the disclosed tackifying agents are combined withthese colloidal suspensions the severe binder migration problemsassociated with colloidal binders such as colloidal silica are overcome.The addition of the tackifying agent imparts good suspensioncharacteristics to the binder. Additionally, the preferred binder of thepresent invention may be sprayed directly onto hot surfaces thuseliminating long delays in insulating furnaces requiring complete cooldown. Finally, the novel binder claimed herein reduces shrinkage of thecoated fibers thus improving their refractory characteristics. Becauseshrinkage is minimized, failure of the insulation layer due to cracks inthe layer is reduced. Prior art binders, especially colloidal silicabinders without the tackifying agent disclosed herein required gellingof the colloidal silica. Otherwise, serious binder migration to thesurface of the insulation layer would occur.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with the present invention, an excellent binder system hasbeen discovered which preferably comprises chromium aluminum phosphateand a suitable tackifying agent. The chromium aluminum phosphatecompound preferred in the present invention may be characterized by thefollowing generalized formula: Al₂ O₃.3P₂ O₅.xCr₂ O₃ wherein x rangesfrom about 0.1 to 10.

The term "tackifying agent", as used with respect to the presentinvention, is meant to define those substances which impart tackiness oradhesive properties to the binder system of the present invention.Generally, such tackifying agents will be inorganic in nature. Hightemperature resistant clays, especially montmorillonite clays(preferably the western bentonite variety) have been found to be usefulin the present invention. Sodium and calcium based clays such assouthern bentonite may also be used as a tackifying agent. Whatevertackifying agent is used should, of course, be compatible with theoverall binder system and its intended use with regard to both chemicaland physical properties. It is especially important that the tackifier,in the quantity used, not produce fluxing of the binder-fiber system atelevated temperatures.

While the inventive binder system may be made according to any suitablemethod known to those skilled in the art, the superior binder system ofthe present invention is preferably made according to the generalizedprocess described herein below.

A mixture of phosphoric acid solution, an appropriate fluxing agent suchas boric acid, and bentonite clay are stirred together and then heatedto a temperature (typically 100° F.) sufficient for the ingredients toreact with one another. Although varying concentrations of phosphoricacid may be used, in the present invention 75% or higher phosphoric acidsolution is preferred. As used herein, the term appropriate fluxingagent is meant to include those substances which will impart addedstrength to the inventive binder system by improving glassy bonding atelevated temperatures. While boric acid is a preferred fluxing agent,other inorganic metal salts such as sodium carbonate, magnesiumchloride, magnesium nitrate, calcium carbonate, cobalt oxide, and othersmay be employed.

After heating to approximately 100° F. the above reaction mixture iscombined with a hydrated alumina. The preferred alumina is a hydratedalumina such as that currently marketed by ALCOA under the trademarkC-31. The mixture is then heated to a temperature of approximately 180°F. at which time chromic oxide (Cr₂ O₃) is added. The preferred chromicoxide has a specific gravity of 5.1 and is added in a percent, byweight, of 1.26% of the total mixture weight. At this point the reactionbecomes exothermic, the temperature rises to approximately 238° F., thevolume of the mixture approximately doubles, and the reaction iscomplete. The solution is allowed to cool and an inert carrier such aswater is added to adjust the specific gravity of the binder to 1.70 atroom temperature.

The above ingredients are used in the present invention at the followinggeneral, preferred, and most preferred weight percentage levels basedupon the total weight of the binder.

                  TABLE                                                           ______________________________________                                                                         Most                                         Ingredient   General    Preferred                                                                              Preferred                                    ______________________________________                                        Phosphoric Acid                                                                            60-90      75-85    78.8                                         Solution                                                                      Fluxing Agent                                                                              0-5        2-4      2.87                                         Tackifying   1-8        1-5      2.87                                         Agent                                                                         Hydrated Alumina                                                                            5-30      10-20    14.2                                         Chromium Compound                                                                          0.5-5      1-2      1.26                                         ______________________________________                                    

While chromic oxide Cr₂ O₃ has been disclosed, other suitable chromiumcompounds include solutions of chromic acid (H₂ CrO₄), and chromium saltsolutions such as magnesium chromate, which can be converted underappropriate temperature conditions to chromia (Cr₂ O₃), may be added.

Similarly, while alumina has been disclosed as the preferred metal oxideto be reacted with the phosphoric acid solution, magnesium oxide orother suitable metal oxides may be reacted with the phosphoric acidsolution without departing from the scope of the invention.

An alternate binder formulation includes colloidal silica combined witha tackifying agent. Preferably NALCO 1115 Colloidal Silica of the 4millimicron size Sol containing 15% solids is used. Approximately fortypounds (40 lbs) of this colloidal silica was mixed with 1.25 pounds ofwestern bentonite to form a tacky binder.

The addition of a montmorillonite clay such as western bentonite to thebinder matrix has been found to produce a tacky binder which adheres tovirtually any surface. While Western Bentonite is preferred, any of theclays selected from the Montmorillonite types of clay minerals have beenfound to be suitable tackifying agents.

The binder of the present invention may be advantageously employed withrefractory fibers in a spray-on process. That is, refractory fibers maybe directed from a spray gun while at the same time being coated withthe binder of the present invention. Such application of refractoryfiber and binber has been found to raise the operating temperature ofthe refractory fibers above their normal rated temperature. Forapplication of refractory fiber to a surface such as a furnace wall, thebinder is diluted, preferably with water, in a 4.5 to 1 water to binderratio by volume. This ratio may range from 2 to 1 to 15 to 1 by volume.The diluted binder is preferably stirred in the dilution container toensure a homogenous mixture during the spraying process. The dilutedbinder may be sprayed with the fiber onto a surface in a ratio whichdepends upon binder dilution. For the preferred chromium aluminumphosphate binder disclosed above, a ratio of 1.75 lbs. of fiber to 1.0lb. binder is preferred.

Examples of different binder formulations and concentrations are listedbelow:

Binder 1/21 was a mixture of NALCO AG 1115 colloidal silica marketed byNALCO Chemical Corp. of 4 millimicron size particles containing 15%solids. Forty pounds of this solution was mixed with 1.25 lbs. ofwestern bentonite. The resulting binder was sprayed in a 30% binder 70%alumina-silica-zirconia fiber ratio by weight and cured at 1000° F. A 12lb/ft³ composite material resulted having good integrity.

Binder #2 was a mixture of the chromium aluminum phosphate binder matrixwith bentonite clay and boric acid in the most preferred formulationpreviously disclosed. This binder concentrate was diluted 4 to 1 byvolume with water and sprayed with alumina-silica-zirconia fiber in aratio of 14% binder to 86% fiber by weight. This composite was thencured at 1000° F. and produced a 12 lb/ft³ material.

Binder #3 was a mixture of 1/2 lb. colloidal alumina, 2 lbs. of -325mesh Al₂ O₃ particles and 11/4 lbs. bentonite in 40 lbs. of water. Thisbinder concentrate was sprayed with alumina-silica-zirconia fibers in aratio of 40% binder to 60% fiber. The resulting material was cured at1000° F. and produced a material which did not adhere well to the bricksubstrate and did not produce material or sufficient integrity to allowdensity to be accurately measured.

Binder #4 was a mixture of 11 lbs. of Kaolin Clay, 3/4 lbs. westernbentonite and 30 lbs. of water. This binder mixture was too viscous tobe sprayed.

Binder #5 was a mixture of the chromium aluminum phosphate binder matrixwith bentonite clay and boric acid in the most preferred formulationpreviously disclosed. This binder concentrate was diluted 10 to 1 byvolume with water and sprayed in equal amounts by weight withalumina-silica-zirconia fiber. The resulting material was cured at 1000°F. and produced a material of 15 lb/ft³ density.

Binder #6 was a mixture of the chromium aluminum phosphate binder matrixith the bentonite clay and boric acid in the most preferred forumulationpreviously disclosed. This binder concentrate was diluted 15 to 1 byvolume with water and sprayed in a ratio of 45% binder to 55%alumina-silica-zirconia fiber by weight. The resulting material wascured at room temperature and produced a material of 13 lbs/ft³ density.

Binder #7 was a mixture of the chromium aluminum phosphate binder matrixwith bentonite clay and boric acid in the most preferred formulationpreviously disclosed. This binder was diluted 15 to 1 by volume withwater and sprayed in a ratio of 36% binder to 64%alumina-silica-zirconia fiber by weight. The resulting material wascured at 1000° F. but did not produce a material of sufficient integrityto obtain a density measurement.

Binder #8 was the colloidal silica binder disclosed as binder #1 withoutthe bentonite clay. The binder was sprayed in a 60% binder to 40% fiberratio by weight with alumina-silica-zirconia fibers. The resultingmaterial was cured at 1000° F. but did not produce a material ofsufficient integrity to obtain a density measurement due to bindermigration.

Binder #9 was a mixture of bentonite clay and water in a ratio by weightof 10% bentonite to 90% water. This binder did not bond with thealumina-silica-zirconia fiber.

A comparison of characteristics of the binder formulations is givenbelow:

    ______________________________________                                                         Integrity with                                                        Binder  Fibers after                                                                              Binder                                                    Tacky   Curing      Migration                                        ______________________________________                                        Binder #1  Yes       Good        Some                                         Binder #2  Yes       Fair        Very                                                                          little                                       Binder #3  Yes       Fair        Some                                         Binder #4  Yes       Unknown     Unknown                                      Binder #5  Yes       Excellent   Very                                                                          little                                       Binder #6  Yes       Excellent   Very                                                                          little                                       Binder #7  Yes       Fair        Unknown                                      Binder #8  No        Poor        Very                                                                          heavy                                        Binder #9  Yes       None        Unknown                                      ______________________________________                                    

A comparison of shrinkage between the materials of binders #1, 2, 3, 5,and 6 was made with each other and with the fiber alone. After beingexposed to a temperature of 2400° F. for four hours the binder--fibercomposites exhibited the following shrinkage characteristics:

    ______________________________________                                                    Shrinkage (%)                                                     ______________________________________                                        Binder #1     2.6                                                             Binder #2     1.5                                                             Binder #3     1.6                                                             Binder #5     1.6                                                             Binder #6     1.4                                                             Fiber alone   2.0                                                             ______________________________________                                    

From the above it can be seen that composites formed from the mostpreferred binder concentrate formulation of binder #2, 5 and 6 exhibitedless shrinkage than the fiber alone. While binder #3 exhibited lowshrinkage, the specific binder formulation exhibited only fair bondingcharacteristics.

A comparison of curing temperatures was made for binder #5. That is, thematerial made under binder formulation #5 was cured at 250° F., 350° F.and 450° F. Each material was then placed in a 90% humidity chamber for72 hours with the following results: the material cured at 250° F.received 31% added moisture and was wet and soft suggesting incompletecuring; the material cured at 350° F. received 18% added moisture andwas somewhat wet and soft, but was marginally acceptacle; and thematerial cured at 450° F. received 13% added moisture and was hard andheld its integrity. Thus the curing temperature has a lower limit ofabout 350° F. with a preferred curing temperature of about 450° F. ormore.

In order to determine the preferred limits of dilution and binder tofiber ratio for the most preferred chromium aluminum phosphate binder,the formulations of binders #2, 5, 6 and 7 were tested. The fiber-bindercomposite materials were compared with respect to the weight ratio ofthe original binder concentrate to fiber weight ratios in light of theintegrity observed in the resulting material. The ratio of the weight ofbinder concentrate to fiber was 4.86% for binder #2, 14.52% for binder#5; 8.33% for binder #6; and 5.72% for binder #7. The materials producedby binders #5 and 6 had excellent integrity while the material frombinders #2 and 7 had fair integrity. This suggests that the preferredbinder concentrate to fiber weight ratio should be at least 6%. Thus,the binder dilution and diluted binder to fiber weight ratio should beadjusted accordingly.

What is claimed is:
 1. A method for forming a layer of refractory fiberson a surface comprising the steps of:spraying a stream of said fiberstoward a surface; coating said sprayed fibers with a tackifier and aninorganic liquid binder including a colloidal suspension of alumina,silica, or zirconia; and curing said coated fibers.
 2. Method accordingto claim 1 wherein said tackifier includes an inorganic material. 3.Method according to claim 1 wherein said refractory fibers includealumina-silica fibers.
 4. Method according to claim 1 wherein saidrefractory fibers include alumina-silica-zirconia fibers.